A flow-control valve assembly is known that has a housing formed with a valve chamber, an inlet port and an outlet port both opening into the valve chamber, and a valve seat at one of the ports. A valve body is displaceable in the valve chamber between a closed end position engaging the seat and blocking flow between the ports and an open end position clear of the seat and permitting flow between the ports through the chamber. The valve body is actuated by a control body in a control chamber and having a pair of oppositely directed faces. This control body subdivides the control chamber into a pair of compartments at the respective faces. A link interconnects the control and valve bodies for joint movement to increase the volume of one of the compartments while decreasing that of the other compartment as the valve body moves from the open to the closed position and to decrease the volume of the other compartment while increasing that of the one compartment as the valve body moves from the closed to the open position.
A sump at substantially lower pressure than the inlet port can be connected by appropriate control valves to either compartment, and the other compartment can be connected through a conduit to the inlet port. Thus one of the compartments of the control unit can be depressurized and the other pressurized to move the valve body in the appropriate direction.
Such a valve assembly, as described in German Pat. No. 3,131,860, is typically used in hydraulic systems and can work at pressures in the 300 bar range. Normally it is set up as nothing more complex than a shutoff valve, as accurate control of the flow section of the passage it forms is quite difficult.
The main difficulty with such arrangements is that the considerable pressure differential across the control body is extremely difficult to seal adequately. Leakage across the control body allows the valve body to move somewhat, thereby making the valve fairly useless. In addition such devices are not employed in water lines and the like where any grit or other impurities might be present in the liquid whose flow is being controlled and which is being used to actuate the valve.
It is known to use the pressure of the fluid being controlled to close the valve, while using an externally generated force to open it. Such a valve is a pure on/off or shutoff valve in which the flow cross section while open can be set by an external screw-type mechanism. While in use one compartment is always pressurized at the inlet pressure, so that when the valve is closed there is once again a very large pressure differential at this location, leading to leakage. The external force can even be a simple electromagnetically generated one, allowing accurate adjustment of the valve, while working very slowly in view of the high pressure it is working against
A further system is known from German Pat. No. 2,750,510 which works as a proportional valve that can be adjusted dynamically. The control body in this arrangement is braced against a spring, so that once again there is a considerable pressure differential between its pressurized and its nonpressurized side.